RationaleIn soft chemical ionization mass spectrometry, analyte ions are produced via ion–molecule reactions in the reactor. When an electric field E is imposed, the ion drift velocity v_d determines the reaction time and the effective ion temperature. Agreement between experimental ion mobilities and theoretical predictions confirms the accuracy of the ion residence time measurement procedure.MethodsA selected ion flow‐drift tube (SIFDT), an instrument with a chemical ionization source, was used to produce protonated aldehydes and selectively inject them into the resistive glass drift tube filled with He. Arrival‐time distributions of ions were obtained using the Hadamard modulation. Reduced ion mobilities were then obtained at a pressure of 2 hPa in the E/N range of 5–15 Td. Theoretical ion mobility values were calculated using two methods: hard‐sphere approximation and trajectory modelling.ResultsThe measured mobilities of three saturated and three unsaturated protonated aldehydes do not show substantial variation across the studied E/N range. Effective temperatures calculated using the Wannier formula from measured gas temperatures ranged from 300 to 315 K. Experimentally obtained values of the near‐zero‐ E/N‐reduced ion mobilities agree with both methods of calculations typically within ±3% standard deviation (maximum ±5%).ConclusionsThe experimental SIFDT values of reduced mobilities in He of protonated aldehyde molecules generated from a chemical ionization source are in close agreement with two different theoretical methods based on the density functional theory calculations of ion geometries and partial atomic charges. Besides its fundamental importance, the ion mobility results validate the correct operation of the drift tube reactor and the ion residence time measurement procedure. Diffusion losses can also be determined from these results.